"Sue" on display at the Field Museum of Natural History in Chicago, Illinois. Photographed by Evolutionnumber9
and licensed under CC BY-SA 4.0.
and licensed under CC BY-SA 4.0."This is the theropod."
— Gregory S. Paul
There's no doubt that Tyrannosaurus rex is, by far, the most famous of the stock dinosaurs, seen as both the most majestic and most terrifying of them all. Let's learn a bit more about this incredible animal, shall we?
- Name: Whereas most dinosaurs are known outside paleontological circles only by their genus, T. rex is known by its full binomium. Tyrannosaurus, the genus name, comes from the Greek tyrannos, meaning "tyrant", and sauros, meaning "lizard". Rex, the specific name, meanwhile, is Latin for "king", therefore the whole name translates to "tyrant lizard king". As per the rules of binomial nomenclature, the genus name should be spelled with a capital T, whereas the species name should be spelled with a lowercase r. And the correct abbreviation of the name is T. rex, not T-rex and especially not T-Rex. Early fossils were described under the names Dynamosaurus imperiosus and Manospondylus gigas — both of them are now considered invalid synonyms. A number of other names have also become invalid synonyms of Tyrannosaurus over the years, including Nanotyrannus, Stygivenator, and Dinotyrannus. Most of these were formerly believed to be dwarf T. rex relatives for their slender frames and narrower snouts until scientists realized that juvenile T. rex simply lacked the heavy skulls and robust bodies of the grown-ups. In 2024, a second species of Tyrannosaurus was named from a partial skeleton found in New Mexico, dubbed T. mcraeensis ("tyrant lizard of the McRae Formation"). This species seemingly lived a few million years earlier than T. rex and may have been ancestral to it.
- Discovery: The first T. rex fossils were found in 1874, but were not recognized as T. rex until many decades later. The species was officially described by Henry Farfield Osborn in 1905, based on a partial skeleton found by Barnum Brown in Hell Creek, Montana. In the same paper describing T. rex, Osborn described the fossils of another large carnivore, found in Wyoming, as Dynamosaurus imperiosus ("ruling power lizard"), but later realized that the two belonged to the same species (the rules of scientific naming say the first name is gets priority, and T. rex beat Dynamosaurus to seniority by only a few pages). The most complete T. rex skeletons were found in 1990 and 1992, and were dubbed as "Sue" and "Stan". These two fossils helped us get a much more accurate image of what the species was like.
- Classification: It was generally believed for most of the 20th century that Tyrannosaurus was closely related to other giant, carnivorous theropod dinosaurs, such as Allosaurus and Megalosaurus, as part of the group Carnosauria. However, studies in the 90s revealed T. rex to in a fact be a part of the group Coelurosauria, which includes many small, feathered theropods, like Ornithomimus, Compsognathus, Velociraptor, and Archaeopteryx as well as modern birds. Within Coelurosauria, T. rex was part of the family Tyrannosauridae. Notable tyrannosaurids include Tarbosaurus (sometimes considered as an Asian Tyrannosaurus species, T. bataar), Albertosaurus, Daspletosaurus, Nanuqsaurus, Teratophoneus, and Alioramus. They were part of the superfamily Tyrannosauroidea, whose notable members include Alectrosaurus, Bistahieversor, (both sometimes considered true tyrannosaurids) Dryptosaurus, Moros, Eotyrannus, Dilong, Stokesosaurus, Yutyrannus, Guanlong, and Proceratosaurus. Also possibly within Tyrannosauroidea are the megaraptorans (named for their most famous member Megaraptor), a group of theropods primarily found in the Southern Hemisphere with slender skulls and massive arms ending in talon-like claws likely used in killing prey — though they may have instead been carnosaurs or a more primitive group of coelurosaurs.
- Time period: T. rex lived at the very end of the Cretaceous period, 68-66 million years ago (a time known as the Maastrichtian age). As one of the very last non-bird dinosaurs, it was around to experience the infamous asteroid collision that ended the Mesozoic era. Any depiction of T. rex earlier than that is, therefore, inaccurate. T. mcraeensis lived between 3 and 5 million years earlier than T. rex.
- Range: T. rex was an exclusively North American species. During the Late Cretaceous, North America was divided into two smaller continents by a shallow sea named the Western Interior Seaway; T. rex lived on the western continent, dubbed Laramidia. Its fossils have been found throughout Laramidia, ranging from Alberta and Saskatchewan in the north to Texas and New Mexico in the south, possibly even as far as Sonora, Mexico. However, its closest relative was was the aforementioned Tarbosaurus, which lived in East Asia at the same time. It is theorized that because of how closely related it was to its Asian counterpart, T. rex's ancestors likely originated from Asia and crossed over the Bering Strait land bridge into North America, eventually taking over most of Laramidia. Albertosaurus, which lived in Laramidia before this, became extinct shortly after T. rex appeared, lending credence to this theory. This, however, came into question with the naming of T. mcraeensis, raising the possibility that T. rex''s ancestors actually originated in North America and instead entered Asia.
- Size: T. rex was famous for being one of the largest theropods, rivalling notorious contenders Giganotosaurus and Spinosaurus in length and likely being heavier than either. The "Sue" skeleton is 12.3 to 12.8 m long (the inaccuracy comes from a few missing vertebrae), about 4 m tall at the hip, and is estimated to weigh 8.4 to 14 metric tons (for comparison, that's 2 to 3 times the weight of an African bush elephant). It was also much bulkier in real life compared to the athletic, "cut"note manner that fiction and older reconstructions tend to depict it with. As a result, it probably wasn't very fast (current estimates of its top speed generally agree on somewhere between 18-24 km/h), but given that most of its usual prey was also large and bulky, it didn't have to be.
- Posture: Early restorations depicted T. rex in a kangaroo-like tripod posture, dragging its long tail on the ground like Godzilla. However, thanks to more complete skeletons, now we know that it held its body horizontally, balancing its body and huge head with its powerful tail.
- Big head: T. rex had a massive head, even in comparison to other big, predatory theropods. Its mouth was full of sharp teeth, up to 20 cm (8 inches) long, sometimes dubbed "killer bananas" because of their size and shape. Its bite force is estimated to be about 8,000 pounds, stronger than any other known land animal, which it needed to crush the bones of the large, often armored dinosaurs it ate.
- Puny arms: One of the most iconic, and most ridiculed, traits of T. rex is its tiny arms (though many theropods had proportionately even smaller arms). They were two-fingered with sharp claws on them, and the palms faced inward (rather than downward, as often erroneously depicted). The reason for the arms' small size is mainly practicality; large arms would have gotten in the way of T. rex's bite, which was the dinosaur's primary killing tool. Thus, whereas other predators have clawing weapons to complement their teeth, T. rex put all its power into its jaws. But even in spite of their size, the arm bones show signs of large muscle attachment (a hallmark of coelurosaurian ancestry) and thus, they were very strong and capable of lifting up to 200 pounds (90 kilograms). Because of this, T. rex might have used them to hold onto struggling prey while it dispatched it with its jaws. In addition, they could have also been used to help lift T. rex up from a sleeping position when it was waking up, hold onto a mate while copulating, or even to pick up its offspring the same way a crocodile uses its jaws to carry its babies.
- Hunter or scavenger?: Though the T. rex is typically portrayed as a hunter in media, there is an infamous debate among paleontologists as to whether or not T. rex was actually a scavenger instead, popularized by paleontologist and Jurassic Park dinosaur consultant Jack Horner. While proponents of the "scavenger" theory point to T. rex's slow speed, ability to crush bone, and acute sense of smell, the overwhelming majority of experts believe T. rex was primarily a hunter, as evidenced by its potential prey also being fairly slow, its superb hearing and eyesight, its massive caloric requirements, and the existence of plant-eating dinosaur fossils with T. rex bite marks on them that show signs of healing (indicating they survived a T. rex attack). However, T. rex would have also definitely eaten any carrion it came across, and it's possible that large, adult Tyrannosaurus would regularly Kill Steal the hunts of other carnivores, including rival T. rex. Modern predators, like tigers and great white sharks, also do this, especially in old age. That said, T. rex regularly hunted some formidable prey — heavily armored animals like Triceratops and Ankylosaurus, as well as herd-moving behemoths like Edmontosaurus and Alamosaurus. On the other hand, young Tyrannosaurus, with their slenderer bodies and narrower snouts, hunted faster-moving and more delicate mid-sized animals like Ornithomimus, Thescelosaurus, Leptoceratops, and Pachycephalosaurus. Bite marks on T. rex bones attributable to other T. rex also suggest it may have even been cannibalistic. T. rex most likely hunted by ambush, using a short burst of speed to catch prey before using its massive body to knock the victim over and finishing them off with their powerful jaws.
- Feathers or scales: Historically, T. rex was portrayed with lizard- or crocodile-like scaly skin. But with the discovery that many dinosaurs, particularly coelurosaurs (including the tyrannosauroids Yutyrannus and Dilong), were feathered, it's been suggested that T. rex was too, basically looking like a giant toothy bird. However, fossilized skin impressions of T. rex and other tyrannosaurids show that most of its body was, indeed, scaly (or at least, extremely sparsely feathered); the only place that was potentially fully feathered was its back, similar to the mane of a lion. As T. rex was a large animal living in warm climate, it likely did not need the extra insulation from a thick coat of fluff, just as similarly-sized mammals like rhinoceroses and elephants are sparsely haired. Some still speculate that it had downy feathers as a hatchling, when it was still small enough to need insulation, and eventually lost these as it reached a certain age and size, much like a baby penguin molting away its birth feathers as it becomes an adolescent.
- Sound: T. rex is typically depicted in media with a Mighty Roar; ever since Jurassic Park, everyone "knows" what that roar sounded like. However, there is actually little evidence that T. rex could roar the same way a lion or a bear could, since it lacked the vocal organs that allow mammals to do so. It may have instead produced low-pitched rumbling, hissing, and bellowing, similar to crocodilians, large flightless birds (like ostriches and cassowaries), and bitterns. It still would've sounded pretty damned impressive, although it may have been bit underwhelming for those used to the movies. Basically, imagine the sound an alligator makes, then imagine if that alligator was 40 feet long and weighed over 8 tonnes. Furthermore, the vibrations produced by a communicating T. rex may have been deep and powerful enough to have been felt when they rippled through the air, possibly even creating small tremors in the earth.
- Senses: T. rex had extraordinary senses of smell and hearing. Analysis of the braincase in fossilized skulls shows that it had large olfactory bulbs and a long cochlear duct capable of receiving low-frequency sounds. These traits would have been advantageous both as a predator (tracking and listening for prey) and as a scavenger (finding carrion from a great distance and detecting approaching rivals). However, contrary to what Steven Spielberg and Michael Crichton would have you believe, T. rex also had excellent eyesight that would've let the creature spot objects from as far as six kilometres with thirteen times the detail. It had large eyes facing forward connecting to big optical lobes, and a relatively narrow snout, allowing for binocular vision. This is probably T. rex's most unique trait, as almost no other dinosaur had vision like this (most other large, predatory theropods have poor eyesight and relied mainly on their sense of smell). This ability supports the idea that T. rex was a hunter, as binocular vision is beneficial when chasing prey.
- Intelligence: Most early depictions of T. rex have them as solitary brutes with little in the way of brainpower or social ability. More recent studies have shown that T. rex had a bigger brain-to-body ratio than previously thought, being smarter than earlier species of giant theropods. In fact, some studies (albeit controversial ones) suggest T. rex may have had an intelligence similar to wolves, or even baboons, which would have made it one of, if not the, smartest non-avian dinosaurs. Given its adaptations and how its common prey items included some heavily armored and highly social animals, it's likely T. rex would need the intelligence to be able to take down its prey with different strategies and know when to throw in the towel and eat a pre-killed carcass instead. Some scientists have also proposed that T. rex hunted in packs, but the idea of such coordinated behavior in predatory dinosaurs is controversial, as birds of prey and crocodiles typically do not hunt socially.
- Neighbors: In art and fiction, T. rex is often portrayed living alongside dinosaurs it didn't coexist with in reality. The most egregious cases show it alongside Jurassic dinosaurs like Stegosaurus, but even authors who try to do their homework often slip up by showing T. rex coexisting with Late Cretaceous North American dinosaurs that lived about 5-10 million years earlier in the Campanian age, like Parasaurolophus and Styracosaurus. Dinosaurs that did live with T. rex in Maastrichtian-aged North America 68-66 million years ago include Triceratops, Ankylosaurus, Edmontosaurus, Alamosaurus, Dakotaraptor, Pachycephalosaurus, Ornithomimus, Torosaurus, Denversaurus, Thescelosaurus, Leptoceratops, Anzu, Acheroraptor, Pectinodon, and Trierarchuncus. With the possible exception of "Nanotyrannus", no other tyrannosaurs shared T. rex's ecosystem, making it the undisputed apex predator of its environment (the next largest predator Dakotaraptor was 4-6 m long and weighed 220-350 kg, making it far too small to compete with an adult T. rexnote ). Also found in T. rex's habitat were the giant pterosaur Quetzalcoatlus, the amphibious reptile Champsosaurus, small mammals (such as the semiaquatic marsupial relative Didelphodon and the possible early primate Purgatorius), birds, crocodiles, turtles, snakes & lizards, amphibians, and a variety of fish (including freshwater sharks and rays, gars, bowfins, sturgeons, and paddlefish). It also would've coexisted with very large mosasaurs (around the same size as T. rex), but these were probably limited to coastal and marine waterways.
- Environment: The prehistoric North American range of T. rex was divided between a swampy subtropical forest to the north and arid inland plains to the south, with the nascent Rocky Mountains to the west. These environments supported different faunas. The former, known as the Lancian fauna and most famously represented by the Hell Creek Formation of Montana, was dominated by Triceratops, Edmontosaurus, and Ankylosaurus, while the latter, the less-studied Sanjuanian fauna (known primarily from the Ojo Alamo Formation of New Mexico), was dominated by Alamosaurus, Torosaurus, and Quetzalcoatlus. Most of T. rex's contemporaries mentioned above are Lancian, with many having Sanjuanian counterparts — Ojoraptorsaurus instead of Anzu, Dineobellator instead of Acheroraptor, Glyptodontopelta instead of Ankylosaurus, Ojoceratops instead of Triceratopsnote , and Kritosaurus instead of Edmontosaurus. There are also some species without northern counterparts, like Alamosaurus and a currently unnamed hadrosaur similar to the Campanian-aged Corythosaurus.
